Neonates are more susceptible than adults to a variety of infectious diseases including bacterial, viral, and fungal infections, and human infants respond poorly to vaccination. Although this has been attributed to immaturity of the immune system, little is known regarding the development of cellular and humoral immune responses in neonatal humans or nonhuman primates. Sequential development of immune tissues is difficult to examine in humans, so here we will track the development of immune cells and responses of neonatal macaques from birth through adolescence for comparison with adults. Our prior macaque studies suggest the newborn primate mucosal immune system is much more developed and competent, and develops at a faster rate than the systemic immune system. Further, recent discoveries of key regulatory cells including Th17 and Treg cells that stimulate or suppress immune responses respectively may develop at different rates in mucosal and systemic tissues of neonates, and may affect the way infants respond to immunization or infection. We hypothesize that sequential development of key regulatory immune cells occurs in normal neonatal tissues, particularly in mucosal sites, as these are more frequently exposed to environmental antigens. Since the mucosal immune system is more developed at birth, we also hypothesize that mucosal immunization strategies may result in improved vaccine responses in newborns and neonates. To test these hypotheses, we will examine and compare the emergence and development of CD4+ Th1, Th2, Treg, and Th17 cells, CD8+ T cells, B, NK and antigen presenting cells (APC), their na?ve/memory status, activation, and homing receptor expression, in both mucosal and systemic lymphoid tissue compartments of developing infants for comparison with adults. Soluble mediators including cytokines, chemokines, and antibody production will be quantified in plasma, mucosal secretions, and cell supernatants at different stages of development. Finally, cellular and humoral immune responses will be compared between infants vaccinated mucosally and systemically, and at different stages of neonatal development to optimize immunization strategies for infants. Immune responses to primary and secondary immunizations will be compared to determine whether mucosal vaccinations result in superior vaccine responses over systemically immunized macaques. Combined, these studies may explain many of the mysteries of neonatal immunology, and will have profound significance for human pediatric vaccination strategies.